The invention relates to a method for performing a plasma chemical vapour deposition process, comprising the step of providing an apparatus comprising a mainly cylindrical resonator being provided with an outer cylindrical wall and an inner coaxial cylindrical wall defining therebetween a resonant cavity operable at an operating frequency and extending in a circumferential direction around a cylindrical axis of the inner and outer cylindrical wall, wherein the outer cylindrical wall includes an input port connectable to an input waveguide, and wherein the inner cylindrical wall includes slit sections extending in a circumferential direction around the cylindrical axis, the method further comprising the step of operating the apparatus at an operating frequency.
European patent publication EP 2 605 267 in the name of Draka Comteq B.V. discloses an apparatus for manufacturing an optical fiber. In the plasma activated chemical vapour deposition (PCVD) process, deposition is performed on the inside of a substrate tube. In this process, the resonator is fed by a microwave source (typically a magnetron). Inside the substrate tube, the microwave power creates a plasma which activates a reaction resulting in deposition of thin quartz layers within the substrate tube. The substrate tube and resonator are placed inside a furnace.
When upscaling the PCVD process to applications for manufacturing larger substrate tubes, especially tubes having a larger diameter, e.g. for the purpose of increasing productivity, there is a need for a high degree of rotational symmetry in the quartz deposition, for a reduced chance of mode hopping causing plasma instabilities and inducing plasma flickering phenomena, and for minimal axial near-periodical variation in thickness and refractive index due to microwave oscillations caused by interaction with the surrounding such as the PCVD furnace. The axial near-periodical variation may have strong impact on some resulting fiber quality parameters such as the attenuation (OTDR traces) and/or the uniformity of mode-field diameter for single mode fiber and/or the uniformity of the alpha-value for graded index multi-mode fiber.